Heart disease currently is a leading cause of death and disability among adults. One type of heart disease, atherosclerosis, is particularly common. Atherosclerosis, a disease of the coronary arteries, is characterized by a thickening of the lining of the coronary arteries, from the deposit of fatty substances such as cholesterol. When this occurs, the inner diameter of the coronary arteries is reduced, and so is the blood flow to the heart. This can cause an insufficient amount of oxygen to reach the heart, particularly during periods of exercise or other exertion. Atherosclerosis can reach a relatively advanced stage before its presence is indicated by symptoms in the patient. However, when symptoms occur, they can be extremely serious. Atherosclerosis can result in angina, or heart pain, or in the most severe circumstances, a heart attack.
There are several widely-accepted treatments for atherosclerosis. One treatment, which is rather invasive, is a coronary bypass. In this procedure, a less-damaged portion of artery, or a segment of synthetic material, is inserted into the patient's coronary artery system to bypass the clogged portion. This typically involves open-heart surgery, which can be extremely traumatic to the patient. Moreover, this type of surgery has a number of serious risks. Finally, such an operation typically requires an extended recovery period.
Another treatment for atherosclerosis is angioplasty, which has been practiced for the last approximately thirty years. Formally known as "percutaneous transluminal coronary angioplasty," or "PTCA," angioplasty involves the insertion of a catheter through an incision in the patient's skin, and advancement of the catheter into the vascular system and toward a blocked coronary artery. When the target is reached, a balloon structure at the distal end of the catheter is inflated and deflated, once or several times, to compress the plaque on the artery wall and stretch the blood vessel, thereby enlarging the diameter of the artery. The enlargement is usually maintained after completion of the procedure by a regulated amount of damage having been done to the elastin and collagen in the vessel.
Angioplasty has some notable disadvantages, however. In many cases, the patient's artery will eventually narrow again. This occurrence is known as restenosis. And in a relatively small percentage of patients, the opened artery will reclose almost immediately after the angioplasty is performed. In such cases, further procedures must be undertaken to reopen the patient's artery; these procedures might include further angioplasty, but when angioplasty has proven to be unsuccessful, open-heart surgery (such as a coronary bypass operation) may become necessary.
It has been found, however, that using an optimum amount of pressure against the artery during the angioplasty procedure will result in the smallest incidence of complications. However, although it may be possible to determine the total amount of pressure being applied to the entire artery wall (for example, by examining the fluid pressure required to inflate the balloon), it is difficult to determine how much pressure has been applied to any particular portion of the blockage.
A treatment related to angioplasty that has become popular within the last three years involves the placement of a reinforcing member known as a "stent" into the damaged coronary artery. A stent is usually a metallic tube-shaped structure that serves as scaffolding, preventing the artery from closing down to its previous size after the procedure is performed. After the procedure is complete, the stent is left in place.
Typically, in a stent implantation operation, a PTCA procedure is performed first, to enlarge the diameter of a clogged portion of a coronary artery. Then, a stent is inserted over a balloon catheter, maneuvered into place, and the balloon is inflated to expand the stent and anchor it into place.
Although many different stent types and configurations are available, a typical stent known as a "Palmaz" stent is formed from a metallic mesh that is deformable to expand and maintain a desired diameter.
Although the placement of a stent may, in many cases, reduce the incidence of acute artery reclosing, and may also reduce the difficulties of restenosis, there are other difficulties and side-effects that can occur when a stent is implanted. First, the stent may perforate the blood vessel if an improper or inconsistent amount of force is applied to inflate the balloon catheter and expand the stent. Second, the position of a stent is difficult to adjust after placement, and it may migrate somewhat in the time shortly after its placement (but typically not later, as the lining of the artery tends to grow over the stent within a period of weeks). Thus, it is particularly important to properly locate and expand the stent when it is first implanted. Third, the existence of a stent in a patient's coronary arteries may result in thrombosis, or the creation of undesired blood clots.
However, it has been found that the accurate placement and deployment of a stent will reduce the incidence of many complications. For example, the inadequate expansion of a stent may cause turbulent blood flow around the stent, resulting in thrombosis. Moreover, in such a case, there would also be additional exposure of the stent material to the blood stream, and any inherent thrombogenicity, or tendency to cause blood clots, of the stent material might be enhanced. Finally, ensuring that the proper amount of pressure is applied to expand the stent will reduce the incidence of perforation. Unfortunately, it is difficult to tell via angiographic fluoroscopy (i.e., X-rays), whether a stent is accurately located and sufficiently expanded with respect to a particular identified blockage.
Consequently, there is a need for a balloon catheter system that permits the accurate determination of whether a sufficient amount of pressure has been applied to adequately expand all parts of the treated coronary artery, or to properly locate and expand a coronary stent. Such a system would reduce the "guesswork" that otherwise is necessary in angioplasty and stent-placement procedures.